Good morning, sorry for my English and for the little experience as a programmer. I managed to install Cumulusmx and its start / stop script inside a raspberry pi with installed raspbian stretch lite. Works well. I reached the graphical interface of cumulus from the browser to associate the station (froggit wh1080) but Cumulus does not detect anything, the station data does not arrive, it is as if there were no connected stations. Tips ?

Just ignore the warnings about signatures and conflicting distribution.

In order to fix dependencies, aptitude must be used to install mono:
$ sudo aptitude install mono-complete=4.0.5.1-0xamarin1

Then you will get a proposal from aptitude to solve the dependencies:

Keep the following packages at their current version:
1) Do NOT accept this solution, when that would install the current version of mono(4.6.2) which has errors (as documented by SpaceHiker).
Respond with "n"

2) Accept the next solution from aptitude to install mono version 4.0.5 with this warning:
Respond with "Y"

3) At the next prompt:
Respond with "Y"

4) At the last prompt:
Do you want to ignore this warning and proceed anyway?
To continue, enter "yes"; to abort, enter "no":
Respond "yes"

Now mono-complete 4.0.5 will be installed.

In order to protect this old mono version (4.0.5) from future upgrades:
$ sudo apt-mark hold mono-runtime mono-complete

E325: ATTENTION
Found a swap file by the name ".mono-xamarin.list.swp"
owned by: root dated: Sun Feb 4 15:28:43 2018
file name: /etc/apt/sources.list.d/mono-xamarin.list
modified: YES
user name: root host name: raspberrypi
process ID: 925
While opening file "mono-xamarin.list"
(1) Another program may be editing the same file. If this is the case,
be careful not to end up with two different instances of the same
file when making changes. Quit, or continue with caution.
(2) An edit session for this file crashed.
If this is the case, use ":recover" or "vim -r mono-xamarin.list"
to recover the changes (see ":help recovery").
If you did this already, delete the swap file ".mono-xamarin.list.swp"
to avoid this message.
"mono-xamarin.list" [New File]
Press ENTER or type command to continue

I think I made a mess,
the error message for the swp file is still present,
if I uninstall mono-complete, the system tells me it's not there, but if I check the mono version, the system tells me that there 4.6.2 is still installed